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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( D ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( D ) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number 000-20288 COLUMBIA BANKING SYSTEM , INC . ( Exact name of registrant as specified in its charter ) Washington ( State or other jurisdiction of incorporation or organization ) Common Stock , No Par Value ( Title of each class ) 1301 A Street Tacoma , Washington 98402-2156 ( Address of principal executive offices and zip code ) Securities Registered Pursuant to Section 12 ( b ) of the Act : COLB ( Trading symbol ) Securities Registered Pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes x No Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes No x ( 253 ) 305-1900 ( Registrant's telephone number , including area code ) Large accelerated filer Non - accelerated filer Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes x No ■ 91-1422237 ( I.R.S. Employer Identification Number ) Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes x No The Nasdaq Stock Market LLC ( Name of each exchange on which registered ) Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or emerging growth company . See definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Accelerated filer Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . X Portions of the Registrant's definitive 2021 Annual Meeting Proxy Statement . Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes No x The aggregate market value of Common Stock held by non - affiliates of the registrant at June 30 , 2020 was $ 2,017,703,718 based on the closing sale price of the Common Stock on that date . The number of shares of registrant's Common Stock outstanding at January 31 , 2021 was 71,633,149 . DOCUMENTS INCORPORATED BY REFERENCE : Part III